Semiconductor laser diodes have numerous advantages. They are small in that the widths of their active regions are typically submicron to a few microns and their heights are usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability.
High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (.about.800 nm) which is near the absorption spectrum of common solid state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid state laser systems.
Universal utilization of semiconductor laser diodes has been restricted by thermally related problems. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated junction temperatures and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the junction increases.
Furthermore, the emitted wavelength of a laser diode is a function of its junction temperature. Thus, when a specific output wavelength is desired, maintaining a constant junction temperature is essential. For example, AlGaAs laser diodes that are used to pump a Nd:YAG rod or slab should emit radiation at about 808 nm since this is the wavelength at which optimum energy absorption exists in the Nd:YAG. But, for every 3.5.degree. C. to 4.0.degree. C. deviation in the junction temperature of the AlGaAs laser diode, the wavelength shifts 1 nm. Accordingly, controlling the junction temperature and, thus, properly dissipating the heat is critical.
When solid state laser rods or slabs are pumped by laser diodes, dissipation of the heat becomes more problematic since it becomes necessary to densely pack a plurality of individual diodes into arrays which generate the required amounts of input power for the larger, solid state laser rod or slab. However, when the packing density of the individual laser diodes is increased, the space available for extraction of heat from the individual laser diodes decreases. This aggravates the problem of heat extraction from the arrays of individual diodes.
One known package which attempts to resolve these thermally-related problems includes the use of a thin, thermally conductive ceramic structure, like beryllium oxide. The ceramic structure includes a plurality of grooves which are cut, etched or sawed therein. A metallized layer extends from groove to groove to conduct electricity therethrough for supplying electrical power to the plurality of laser diodes which are soldered to the metallized layers in the grooves. This type of package is generally disclosed in several U.S. patents to Karpinski including, for example, U.S. Pat. Nos. 5,128,951 and 5,040,187.
However, this known package has several problems. For example, laser diodes typically have an inherent curvature due to the process by which they are made. Placing a curved laser diode in the straight groove of this known package results in additional stress on the laser diode and often an uneven solder bond along the length of the laser diode which can lead to failure. Because the grooves are typically deeper than the laser diodes, it can be difficult to control the location of the emitting surface of the laser diode in this known package. If beryllium oxide is the material used in this package, further problems arise since it is a toxic material and cutting grooves produces airborne dust particles.